1. Field of the Invention
The present invention relates to an image recording material which can be used as an offset printing master. More particularly, the present invention relates to a positive planographic printing plate precursor for use in direct plate formation with an infrared laser, in which an image of the plate can be formed directly by exposing the plate to an infrared laser on the basis of digital signals from a computer or the equivalent.
2. Description of the Related Art
The development of lasers in recent years has been remarkable. In particular, high-output, compact solid-state lasers and semiconductor lasers having an emission area from near infrared to infrared have become readily available. These lasers are very useful as exposure light sources when making a plate directly from digital data from a computer or the like.
Positive planographic printing plate material for exposure to an infrared laser contains a binder resin that is soluble in an aqueous alkali solution, an infrared(IR) dye that absorbs light to generate heat and the like as an essential component. At unexposed portions (image portions), the IR dye and the like serve as a dissolution inhibitor to substantially reduce the solubility of the binder resin by interacting with the binder resin. At exposed portions (non-image portions), the interaction between the IR dye and the like and the binder resin is weakened by the generated heat, wherein the exposed portions are dissolved in an alkali developer to form a planographic printing plate.
However, in such the positive planographic printing plate material, the difference under various conditions of use between resistance to solubility of the unexposed portions (image portions) in a developer and solubility of the exposed portions (non-image portions) in a developer is still insufficient, and there has been the problem that over development or under development is easily caused by variations in conditions of use. In addition, the surface of the planographic printing plate is easily compromised by fine scratches generated by the surface of the planographic printing plate being contacted during handling. Thus, there has been the problem that, even when the surface of the planographic printing plate is only slightly compromised by such fine scratches, solubility of compromised non-exposed portions (image portions) is increased, whereby the non-exposed portions are dissolved at the time of development, scars are left on the surface. That is, printability deteriorates, ink does not properly adhere to the surface of the planographic printing plate, and the appearance of obtained images deteriorates. Moreover, there is an additional drawback in that, because there is the potential for the scarred areas of the surface to reduce performance, it becomes necessary to conduct a re-exposure or prepare another plate precursor and to expose it, whereby labor is needlessly expended.
Such problems are derived from an essential difference in the mechanism by which a plate is made by exposing a positive planographic printing plate material to an infrared laser and the mechanism by which a plate is made by exposing a positive planographic printing plate material to ultraviolet light. In the case of the latter, the positive planographic printing plate material includes as essential components a binder resin that is soluble in an aqueous alkali solution, and an onium salt, quinonediazide compounds or the like. The onium salt and the quinonediazide compounds not only function as dissolution inhibitors by inhibiting dissolution at unexposed portions (image portions) by interacting with the binder resin, but also function as dissolution accelerators by releasing acids upon being decomposed by light at exposed portions (non-image portions), thereby performing dual roles.
In contrast, the IR dye and the like in the positive planographic printing plate material for exposure to an infrared laser functions only to inhibit dissolution of the unexposed portions (image portions), and does not accelerate dissolution of the exposed portions (non-image portions). Therefore, in the positive planographic printing plate material for exposure to an infrared laser, in order to produce a difference in the solubilities of the unexposed portions and the exposed portions, it is necessary to use, as a binder resin, a resin having high solubility in an alkali developer in advance. These results in problems such as weakened resistance to scratches and unstability of the plate precursor before development.
Various strategies have been proposed to inhibit variance in developability caused by scratching of the unexposed portions (image portions), such as disposing a protective layer on a positive recording layer and increasing the thickness of the entire recording layer. However, when a protective layer that is high in film strength and has excellent resistance to scratches is disposed on the positive recording layer, there is the potential for developability to drop. Further, while increasing the thickness of the entire recording layer suppresses defects caused by scratches, there are problems in that sensitivity is reduced and there is a tendency for terminability (release ability) of the dissolution-inhibiting performance at deep portions of the recording layer to be reduced.
Although various dissolution inhibitors have been proposed to improve resistance to developability, few can rapidly terminate the inhibition effect by exposure to light. In order to enhance resistance to solubility of the unexposed portions (image portions) in a developer without reducing the developability of the exposed portions (non-image portions), European Patent No. 950517 discloses a method using a siloxene type surfactant, and Japanese Patent Application Laid-Open (JP-A) No. 10-26851 discloses a method in which sulfonic esters are used as dissolution inhibitors. Such methods may improve resistance to development of the image portions of the recording layer, but do not achieve a sufficient difference in the solubilities of the unexposed portions and the exposed portions to the extent that clear and better image can be formed regardless of variance in the activity of the developer.